The present invention relates to a ghost cancel apparatus for cancelling wavelength distortion or ghost, and which is used in various types of imaging devices such as television receivers and the like, that handle television image signals.
In recent years, a number of methods for ghost cancelling in television systems have been proposed. Almost all of them provide reference signals inserted into the television signals, for ghost detecting and/or ghost cancelling. In receiving, the degree of ghost occurred in TV transmission is detected by a difference between received reference signals and the standard reference signals, and then, variable co-efficients of an equalizer, for example a transversal filter, are adjusted for ghost cancelling. In these procedure, the detection of ghost is the key to correct cancelling. For this purpose, in the television signal now broadcasted in Japan, two types of waveforms are employed. One is "0" pedestal level and another is a GCR (Ghost Cancel Reference) which are inserted alternately onto the 18th and the 281st line of the vertical blanking period. The insertion order of these two waveform is repeated every 8 fields.
In FIG. 1, the image signals that are supplied from an input terminal, are input to an analog/digital (A/D) converter circuit 1 and are simultaneously input to a timing signal generation circuit 4. The image signals that have been input to the A/D converter circuit 1 are converted into digital data, and are supplied to a digital transversal filter 2 (hereinafter simply termed a transversal filter). The timing generation circuit 4 generates timing signals for writing the GCR signals which have been converted into digital data, to a line memory 5 which is a high-speed memory.
Here, the line memory 5 has an 8-line memory block spanning from a line memory 50 to a line memory 57. The GCR signals which have been converted into digital data, are extracted from the transversal filter 2 by reference signal period sample signals which are the timing signals generated by the timing signal generation circuit 4. By addressing signals which indicate to which portion of the 8-line memory block of line memory 5 (line memory 50 to line memory 51) the GCR signals are stored in the line memory 5.
The following is a description of the GCR signals for the detection of waveform distortion such as ghost and the like in input image or video signals, with reference to FIGS. 2(a)-(c). The GCR signals are composed of "0" pedestal signals shown in FIG. 2(b) and the GCR signals shown in FIG. 2(a), and are sent by an 8-field sequence shown in Table 1. For example, field number 1 has the pedestal signal for the 18th line GCR signal sent to it, and field number 2 has the 281st line GCR signal sent to it. The order of storage in the line memory 5 is in accordance with this field sequence.
TABLE 1 __________________________________________________________________________ insertion number 1 2 3 4 5 6 7 8 insertion line 18 281 18 281 18 281 18 281 signal GCR 0 GCR 0 0 GCR 0 GCR __________________________________________________________________________
A GCR subtraction circuit 7 performs subtraction processing (GCR signal--"0" pedestal signal) for the data of the 4-field difference, and generates reference signals shown in FIG. 2(c) from which the burst signals and the sync signals have been cancelled, and supplies the reference signals to a calculation processing apparatus 6 having a microprocessor, a work memory and a program memory. The reference signals that have been read by the microprocessor inside the calculation processing apparatus 6 are added and averaged a required number of times, and the averaged reference signals are differentiated by a differentiating filter not shown in the figure, inside the calculation processing filter 6, and the influence of a direct current (DC) portion is removed.
Then, the microprocessor inside the calculation processing apparatus 6 compares (subtracts) the waveform which serves as the reference, which has been set beforehand inside the microprocessor, with those reference signals, and performs calculation processing on the basis of the comparison results. The results are multiplied by a required multiplication factor and the multiplication factor is applied to the calculation results so that a weighting (compensation coefficient) is set to the transversal filter 2. The set weighting is applied to the transversal filter 2 and ghost is removed (reduced) from the image signals. These image signals which have had ghost removed (reduced), are returned to analog data by a digital/analog (D/A) conversion circuit 3 for output. This processing is performed several of times and ghost is removed from the image signals.
At this time, there can be provided feedback type control and feedforward type control. In feedback control, the image signals which are read from the output side of the transversal filter 2, are used as the basis for feedback control so that the weighting of the transversal filter 2 is successively renewed. In feedforward control, the image signals read from the input side of the transversal filter 2, are used as the basis for determining a weighting of the transversal filter 2. The ghost cancel apparatus shown in FIG. 1 is for feedback type control.
When the image signals are converted into digital data by the A/D conversion circuit 1, the sampling rate for the conversion is high speed (and in the example shown in FIG. 1, the data are sampled at 14.3 MHz), and it is difficult for the data to be directly taken into a low-speed and large-capacity memory used by the microprocessor. In addition, it is also necessary to have simultaneous management for the data which is incorporated for the difference calculation for the obtained data because the GCR subtraction circuit 7 described above first extracts the reference signals by subtraction processing for the data of the 4-field difference.
In order to satisfy these conditions, it is necessary to have an 8-line portion for the high-speed line memory and so the scale of the circuit necessarily large. Alternatively, in order to reduce the scale of the circuit, there is a method in which, of the 8-field sequence, only the 4 fields of the first half, or only the first two fields are, but there is a small number of data taken and the ghost detection sensitivity becomes duller because there is a small number of sampling data. The disadvantage crises that there is a lowering of the processing speed for ghost removal.